A mouse lymphokine that stimulates the production of functional eosinophils in liquid bone marrow cultures has recently been described [Sanderson, C.J., Warren, D.J. & Strath, M. (1985) J. Exp. Med. 162, 60-74]. This factor appears to be specific for the eosinophil lineage in hemopoietic differentiation and is analogous to colony-stimulating factors described for other hemopoietic lineages. In this paper we report that this factor appears to be identical to the B-cell growth factor II described by Swain and Dutton [Swain, S.L. & Dutton, R.W. (1982) J. Exp. Med. 156, 1821-1834]. This conclusion is based on the coordinate expression of the two activities by a panel of alloreactive T-cell clones and lines and on copurification through a series of protein separation techniques. The reason for a single lymphokine's having these widely differing biological activities is unclear, and its duality presents problems in using terminology based on either assay system. For this reason we propose the name "interleukin 4" for this molecule, and we suggest the defining property should be its eosinophil-differentiating activity.